Some pipe manufacturing processes recently implemented are designed to intentionally leave residual moment of given range in a pipe being manufactured depending upon the application to which the pipe is to be put. To line pipe and other similar pipes upon which pressure is exerted from inside, these processes impart a residual moment acting in such a manner as to contract the pipe resisting the pressure from inside. With pipe piles etc. upon which pressure is exerted from outside, their residual moment imparted acts in such a manner as to expand the pipe resisting the pressure from outside. The residual moment is given by adjusting the load applied by the forming rolls on a pipe mill.
The residual moment results from the residual stress that is distributed across the wall thickness of pipe. The direction and magnitude of the residual stress can be estimated from the magnitude of a ring opening or a ring closing resulting from a longitudinal cut made in the pipe. A ring-like sample S of suitable length (approximately 200 mm to 1,000 mm) is taken from a long pipe manufactured, as shown at (a) in FIG. 1. A cut is made along a straight line parallel to the axis of the pipe (as indicated by a dotted line) to release the residual moment. If the residual moment is an expanding one, the cut in the sample S opens up as shown at (b) in FIG. 1. If the residual moment is a contracting one, on the other hand, the cut closes as shown at (c) in FIG. 1. The amount by which the cut opens (+.alpha.) and the amount by which the cut closes (-.alpha.) are collectively known as the magnitude of a ring opening. Even if the magnitude and the direction of the residual moment are the same, the magnitude of a ring opening varies with the outside diameter of pipe. Usually, therefore, the value .+-..alpha./D obtained by dividing the magnitude of ring opening or ring closing by the outside diameter D of pipe is called the normalized magnitude of ring opening .+-..gamma. and used as an index to indicate the magnitude of residual moment.
The normalized magnitude of ring opening, or the magnitude of residual moment has an optimum range depending upon the application to which a pipe is put, as stated previously. It is therefore preferable that the magnitude of residual moment in a pipe should be controlled to such an optimum range during the pipe-making process. Measuring the magnitude, however, has conventionally called for such destructive testing as described previously which comprises cutting off a sample and making a residual-moment-releasing slit in it. Practically, however, such testing has scarcely been conducted because it lowers production yield and consumes much labor. Consequently, the normalized magnitude of ring opening has not been controlled so frequently as to be desired, so its variation has been relatively great. This variation, in turn, has sometimes led to the intolerable deformation of product pipe in service, thereby making the pipe no longer serviceable for the intended application.